The administration of soluble protein antigens via the oral route has been described as a means of inducing systemic immunological tolerance (oral tolerance). The induction of oral tolerance to self-proteins is being explored as a means to treat autoimmune disease in humans. This project is focused on determining the immunolgical mechanisms of oral tolerance induction with the hope that the information gained will result in the development of optimal stategies for oral tolerance induction for the treatment of autoimmune diseases. We have determined the importance of IL-12 for the regulation of two mechanisms of oral tolerance, clonal deletion due to Fas-mediated apoptosis, and the generation of TGF-beta-producing regulatory T cells. We demonstrated that the administration of anti-IL-12 to intact OVA TCR-transgenic mice fed OVA, enhanced both TGF-beta production and apoptosis of antigen-specific T cells. Furthermore, we showed that naive (CD4+/MEL-14hi) OVA-TCR-T cells stimulated with OVA-pulsed dendritic cells in vitro produce 4-5 fold higher amounts of TGF-beta when cultured with anti-IL-12 or anti-IFN-gamma. IL-4 was not required for TGF-beta production, however, it appeared to indirectly enhance TGF-beta production by promoting the growth of TGF-beta producing cells. Taken together, our findings demonstrate that IL-12 and IFN-gamma are important negative regulators of TGF-beta production both in vivo and in vitro. They thus explain the ability of anti-IL-12 treatment to enhance oral tolerance as discussed above. Finally, it was determined that IL-10, primarily by its ability to suppress IL-12 production, and IL-7, primarily by its ability to induce IL-4 production in the T cell priming cultures, can enhance the differentiation of TGF-beta-producing T cells, and that TGF-beta itself can prime T cells for their own production. This latter fact is important, as it suggests that the low level production of TGF-beta by one cell type, such as a T cell, could have dramatic effects on TGF-beta production by another cell, such as a B cell or antigen-presenting cell, the end result being an environment with high levels of TGF-beta that acts to suppress inflammation.